Photovoltaic devices convert electromagnetic radiation into electricity by producing a photo-generated current when connected across a load and exposed to light. The electrical power generated by photovoltaic cells can be used in many applications, including lighting, heating, battery charging, and powering devices requiring electrical energy.
When irradiated under an infinite load, a photovoltaic device produces its maximum possible voltage, the open circuit voltage or Voc. When irradiated with its electrical contacts shorted, a photovoltaic device produces its maximum current, I short circuit or Isc. Under operating conditions, a photovoltaic device is connected to a finite load, and the electrical power output is equal to the product of the current and voltage. The maximum power generated by a photovoltaic device cannot exceed the product of Voc and Isc. When the load value is optimized for maximum power generation, the current and voltage have the values Imax and Vmax, respectively.
A key characteristic in evaluating a photovoltaic cell's performance is the fill factor, ff. The fill factor is the ratio of the photovoltaic cell's actual power to its power if both current and voltage were at their maxima. The fill factor of a photovoltaic cell is provided according to equation (1).ff=(ImaxVmax)/(IscVoc)  (1)The fill factor of a photovoltaic is always less than 1, as Isc and Voc are never obtained simultaneously under operating conditions. Nevertheless, as the fill factor approaches a value of 1, a device demonstrates less internal resistance and, therefore, delivers a greater percentage of electrical power to the load under optimal conditions.
Photovoltaic devices may additionally be characterized by their efficiency of converting electromagnetic energy into electrical energy. The conversion efficiency, ηp, of a photovoltaic device is provided according to equation (2), where Pinc is the power of the light incident on the photovoltaic.ηp=ff*(IscVoc)/Pinc  (2)
Devices utilizing crystalline or amorphous silicon dominate commercial applications. However, many commercially available silicon-based photovoltaic cells are difficult and expensive to produce or exhibit significant performance degradation over the lifetime of the device.